Chapter 11 What is Motion?

Frames of Reference Frames of Reference Whenever you describe something that is moving, you are comparing it with something that is assumed to be stationary, or not moving. Whenever you describe something that is moving, you are comparing it with something that is assumed to be stationary, or not moving. The background or object used for comparison is called a frame of reference. The background or object used for comparison is called a frame of reference. Why is the planet Earth our most commonly used frame of reference? Why is the planet Earth our most commonly used frame of reference?

Measuring Motion Measuring Motion A change in position in a certain amount of time is motion. A change in position in a certain amount of time is motion. When you say something has moved, you are describing motion. When you say something has moved, you are describing motion.

Speed Speed is the rate at which an object moves. Speed is the rate at which an object moves. Speed = Distance ÷ Time Speed = Distance ÷ Time Distance is measured in meters or kilometers. Distance is measured in meters or kilometers. Time is measured in seconds or hours. Time is measured in seconds or hours. Speed is measured in meters per second (m/s) or kilometers per hour (km/hr). Speed is measured in meters per second (m/s) or kilometers per hour (km/hr).

Constant speed When an object does not change it’s rate of speed, it is traveling at a constant speed. When an object does not change it’s rate of speed, it is traveling at a constant speed. When graphed, constant speed is a straight line. When graphed, constant speed is a straight line.

Average Speed Most things do not travel at a constant speed, they either slow down or speed up—This is called average speed. Most things do not travel at a constant speed, they either slow down or speed up—This is called average speed. What does the line between 11:00 and 1:00 indicate? What does the line between 11:00 and 1:00 indicate?

Velocity Velocity is speed in a given direction. Velocity is speed in a given direction. For instance, the SR-71 Blackbird’s fastest speed is 925 m/s. However, it would not be considered velocity unless you specified a direction. For instance, the SR-71 Blackbird’s fastest speed is 925 m/s. However, it would not be considered velocity unless you specified a direction.

Combining Velocities If you were in a plane traveling at 400 m/s with a tailwind of 100 m/s, the two velocities would combine to boost your speed to 500 m/s, although the speedometer in the plane would still only read 400 m/s, people on the ground would record your speed as the greater one. If you were in a plane traveling at 400 m/s with a tailwind of 100 m/s, the two velocities would combine to boost your speed to 500 m/s, although the speedometer in the plane would still only read 400 m/s, people on the ground would record your speed as the greater one. What would your speed be if you turned around and headed into the wind? What would your speed be if you turned around and headed into the wind?

Changes in Velocity Changes in Velocity The rate of change in velocity is acceleration. The rate of change in velocity is acceleration. Acceleration is a change in either speed or direction or both. Acceleration is a change in either speed or direction or both. Acceleration = Final velocity – Initial velocity divided by the time. Acceleration = Final velocity – Initial velocity divided by the time. Acceleration is measured in meters per second per second—m/s 2. Acceleration is measured in meters per second per second—m/s 2.

Circular Motion When an object is going in a circle, it is accelerating because it is constantly changing direction, even though the speed may be constant. When an object is going in a circle, it is accelerating because it is constantly changing direction, even though the speed may be constant.

Momentum Momentum Momentum depends on the mass of the object and the velocity with which it is traveling. Momentum depends on the mass of the object and the velocity with which it is traveling. Momentum = mass x velocity Momentum = mass x velocity Unit is kg m/s Unit is kg m/s

Conservation of Momentum The total momentum of any group of objects remains the same unless outside forces act on the objects. The total momentum of any group of objects remains the same unless outside forces act on the objects. When you hit a billiard ball with another billiard ball and one stops while the other moves, you have transferred the momentum from one to the other. When you hit a billiard ball with another billiard ball and one stops while the other moves, you have transferred the momentum from one to the other.